1. Field of the Invention
This invention comprises a new design scheme for a compensation circuitry for the output voltage pulse of a solid-state Marx modulator. Specifically, design and utilization methods of high voltage compensation cells (HVCCs) are introduced into a high-voltage solid-state Marx modulator for counteracting the voltage droop of its output pulse when the Marx modulator is used in high-power and long-pulse applications. Inductive components regulated by solid-state switches are used in the HVCCs for reliably compensating the voltage droop of the long output pulse (around millisecond order) of the Marx modulator. The invention is also applicable to solid-state Marx pulsers that have a large voltage droop in output voltage pulses.
2. Description of Prior Art
A Marx generator is a device to transform a low charge voltage to a high output voltage pulse. It is a robust, low-impedance source of electric energy that has been utilized in a variety of high-peak-power applications for the past several decades. In recent years, Marx generators using new solid-state switches, e.g. Metal Oxide Semiconductor Field Effect Transistors (MOSFET) and Insulated Gate Bipolar Transistor (IGBT), have been studied for the application of high voltage modulators. This type of modulators, called solid-state Marx modulators or Marx modulators in short, offers an alternative to traditional high voltage (HV) modulators for rf power sources. Their merits are compact size, high-energy efficiency, high reliability, pulse width control and cost reduction. The use of solid-state switches with electrical current interruption capability, in place of spark gap switches or Silicon-Controlled Rectifier (SCR) switches, gives Marx modulators the ability to produce square-shaped output pulses at high repetition rates, and allows the output pulse to change width from one pulse to the next, a capability that gives Marx modulators the ability to adapt rapidly to changing load requirements.
Ideally, the high voltage output pulse by the Marx modulator should have a constant amplitude (or flat pulse) in rf applications. There is no intrinsic limitation for the Marx modulator to generate a flat pulse if its output voltage pulse is short or if the resistance of the Marx modulator's load is high so that their circuit's time constant is much longer than the pulse length. However, a great challenge appears if the Marx modulator has a long output pulse or a small load. The output voltage droops significantly in the latter cases because, when discharging, a Marx modulator can be approximated by a simple capacitor having the capacitance of Cm, if parasitic inductance is small, with the load represented by a resistance RL. The entire modulator circuit together with its load, e.g. a klystron or a magnetron, is a simple discharging RC circuit with a time-constant t=Cm·RL, which determines the severity of the voltage droop at the end of a voltage pulse. A reduction in the time constant or an increase of the voltage pulse duration would lead to a significant voltage reduction at the end of a long voltage pulse, which is generally not acceptable for an rf load such as a klystron. To limit the voltage droop in a narrow range that is required by the load, designers of the Marx modulator need to increase the time-constant t. Since the load is normally not changeable, the total capacitance, Cm, of the Marx modulator need to be increased dramatically, which is equivalent to increasing the total stored electrical energy of the Marx modulator and will incur a great amount of expense.
To circumvent this problem, researchers tried to exploit compensation circuitry to reduce the voltage droop of the Marx modulators (precisely, the Marx cell bank of the Marx modulator, see below) in recent years. The prior art compensation circuitry, named vernier regulator or VC bank, consists of tens of compensation cells (CCs), called vernier cells (VCs) (see papers of G. Leyh, 2005 Pulsed Power Conference, Particle Accelerator Conference 2007 and C. Burkhart, Proceedings of LINAC 2008). These prior art CCs, i.e. VCs, have a similar topology to that of the Marx cells (MCs) within same Marx modulator, but have much lower charge voltage than that of the MCs (see papers of C. Burkhart, Proceedings of LINAC 2008, and G. Leyh, 2005 Pulsed Power Conference, European Particle Accelerator Conference 2004). Therefore, the voltage rating of the components of the VCs is generally much lower than their counterpart in MCs.
For the purpose of discussing the differences between our invention and prior arts, we display a topology of a MC described in above citations in FIG. 1, which has a similar topology to a prior art CC or VC. The charging circuit for the cells is represented by a solid-state charge switch only in FIG. 1, omitting other details in order to highlight the core function of the cell. Isolated switch drives are included in the figure because they are necessary for the solid-state switches exploited by Marx modulators. In this prior art topology, the MC or VC comprises of charge switch 16 together with its isolated switch drive 22 (also called isolated gate drive; same for other isolated switch drives), bypass diode 18, main switch 12 together with its isolated switch drive 20, and energy storage capacitor 14. Bypass diode 18 defaults the current cell if main switch 12 is off during pulse output period. Isolated switch drive 20 and 22 accept the control signals from a control system of the Marx modulator. When charging, main switch 12 is off and charge switch 16 is on. Charge current passes through charge switch 16 to charge energy storage capacitor 14 that is in the next MC in series. During operation, charge switch 16 is turned off while main switch 12 is turned on by the control system through their individual isolated switch drives. Electric energy stored in energy storage capacitor 14 is released to the Marx modulator's load.
When working in a Marx modulator, prior art VCs with the topology in FIG. 1 are turned on one-by-one within the specified pulse duration. Their output voltages are superposed on the negative output voltage of the MC bank, comprising tens of MCs in series, so that the voltage droop (referring to voltage amplitude droop, same meaning below) of the MC bank is compensated. The advantage of using a compensation circuitry in a high voltage modulator is that the Marx modulator can greatly reduce the stored electric energy in the capacitors of its MC bank while still limiting its pulsed voltage droop to the specified range required by the rf load. However, problems exist in these compensation cell designs. First, the output voltage of the compensation cells, i.e. the VCs mentioned above, in series of the MC bank of the Marx modulators superposes on the output voltage of the MC bank, and forms sawtooth shapes (see paper of C. Burkhart, Proceedings of LINAC 2008) on the output voltage pulse of the entire Marx modulator. The charge voltage of each of the VCs must be lowered in order to control the sawtooth height, necessitating a large difference between the charge voltage of a MC and that of a VC. Thus more than one charge source would need to be employed in the same Marx generator. Second, the VCs cannot provide flexible compensation. Only at a pre-set time interval a VC is switched on. Third, many VCs are needed for a Marx modulator with a long output pulse because the VC's storage energy is low and its compensation ability is limited by the low voltage. Fourth, the low charge voltage results in large ohmic loss due to increased charge current, thus diminishing the efficiency, or the energy utilization ratio. All of these problems not only complicate the circuit design, but also increase the cost of the circuitry with uncertain compensation results because a plurality of VCs in the compensation circuitry increase the parasitic inductance and may cause uncontrollable fluctuation during the flat top of the pulsed voltage output. Furthermore, the footprint of the Marx modulator expands as more VCs are added. Each VC is an integrated circuit which is utilized only once during one voltage pulse output.
The present invention provides a new way of compensating the voltage droop of the MC banks of the Marx modulators by enhancing the electric energy storage and utilization of the compensation cells (CCs), while reducing the number of CC units in the Marx modulators, resulting in smaller footprint and lower fabrication cost. Further objectives and advantages of the invention will become apparent from a consideration of the drawings and ensuing description.